The present invention relates to an improved power source switching device, and more particularly relates to improvements in the supply of power provided by a switching device adapted for selectively connecting a load circuit to different power sources such as ordinary and emergency power sources.
One typical example of such a power source switching unit is proposed in Japanese Patent 2-44619 which corresponds to U.S. Pat. No. 4,934,403 and which discloses an extractable type switching device. More specifically, its first contact piece is connected to the first power source, its second contact piece is connected to the second power source and its third contact piece is connected to a load circuit, respectively. When the switch device is registered at its operative position, the device is automatically connected to the power sources as well as the load circuit. As the switch device is extracted out of the operative position, connections are automatically cancelled. The switch device is internally provided with the first contact adapted for separable connection between the first and third contact pieces, and the second contact adapted for separable connection between the second and third contact pieces. A by-pass switch unit is interposed between the first and third contact pieces in order to provide electric and mechanical interlock between the first and second contacts as well as between these contacts and the by-pass switch unit. Use of such a by-pass switch unit effectively prevents concurrent closure of the two contacts and also prevents contact damage due to arc discharge.
In operation of the power source switching device of the above-described proposal, the first or second contact is selectively connected to the third contact piece via manual or magnetic operation in order to shift the power source to be connected to the load circuit. When the switch device is extracted from its housing for maintenance or testing purposes, the first power source is connected to the load circuit in order to avoid undesirable interruption of power supply.
With this conventional system, however, no power can be supplied to the load circuit when the first power source is not in operation.